The present invention relates to the field of communications, and, more particularly, to wireless terminals incorporating antennas and microphones.
Many digital wireless systems in use today utilize a time slotted access system. An information signal (e.g., speech, data, video) is segmented, compressed, packetized and transmitted in a pre-allocated time slot. Time slots can be allocated to different users, a scheme commonly referred to as Time Division Multiple Access (TDMA). TDMA communication systems, such as the Global System for Mobile communications (GSM) in Europe, the Digital-Advanced Mobile Phone System (D-AMPS) system in North America, or the Personal Digital Cellular (PDC) system in Japan, allow a single radio frequency channel to be shared between multiple remote terminals, thereby increasing the capacity of the communication system. Also, Code Division Multiple Access (CDMA) access techniques use a framing structure to gather and transmit information across an air interface.
Time slots may also be allocated between alternating uplink and downlink transmissions, a scheme commonly referred to as Time Division Duplex (TDD). In a TDD system, the transmitter is inactive for a period of time during each frame, which period is of sufficient duration to receive a signal burst.
Such periodic transmission can generate electrical interference at the switching frequency, referred to herein as a fundamental frequency and its harmonic frequencies. For example, in a GSM system, the antenna and associated transceiver circuits are switched on and off at a fundamental frequency of approximately 217 Hz.
The electrical interference can become coupled into the microphone circuitry where it can add an audible buzz to a speech signal that can be heard by a user at a receiving terminal, which can be referred to as “bumblebee” noise in the speech signal. The audible buzz may occur and the fundamental frequency and/or at the harmonic frequencies.
Some existing wireless terminals attempt to suppress bumblebee noise by locating the antenna and the microphone at extreme opposite ends of the wireless terminal (e.g., locating the antenna at the top and the microphone at the bottom of the terminal) to reduce electrical coupling there between. Also analog/digital filters, such as digital interference cancellers, may be used to suppress bumblebee noise.
Accordingly, the need to suppress bumblebee noise may not only constrain the permissible locations of various components within a wireless terminal, it may necessitate the use of filtering circuitry that is specifically and uniquely designed for each different wireless terminal model to compensate for the unique electrical interference characteristics therein.